1. Field of the Invention
This invention relates to a liquid level detection apparatus suited for detecting a liquid level (or height) within a fuel tank of an automobile, and more particularly to an improved resistance plate used in a liquid level detection apparatus in which a float arm connected at its distal end to a float is pivotally moved in accordance with displacement of the float floating on the liquid surface, and a sliding arm slides over the resistance plate in accordance with the pivotal movement of the float arm.
2. Related Art
One conventional liquid level detection apparatus disclosed in Japanese Patent No. 3,833,551 Specification will be described with reference to FIG. 6. As shown in FIG. 6, the liquid level detection apparatus 100 includes a resistance plate 101, and a sliding arm 102 for sliding over this resistance plate 101 in accordance with a pivotal movement of a float arm (not shown). A first slide portion 103 and a second slide portion 104 each comprising an electrically-conductive layer of excellent conductivity are formed on an insulating layer of the resistance plate 101. The first slide portion 103 includes a plurality of first conductive segments 105 arranged at intervals in a direction of sliding movement of the sliding arm 102, and the adjacent first conductive segments 105 are connected together via a resistor 106. The second slide portion 104 includes a plurality of second conductive segments 107 arranged at intervals in the direction of sliding movement of the sliding arm 102, and the adjacent second conductive segments 107 are conductively connected together via a conductive member made of the same material as that of the second conductive segments 107. End lands 110a and 110b are provided respectively at one ends of the first and second slide portions 103 and 104, and a detection output can be obtained between the two end lands 110a and 110b. 
The sliding arm 102 is formed of an electrically conductive material, and includes a first contact portion 120 for sliding over the first slide portion 103, and a second contact portion 121 for sliding over the second slide portion 104. The first slide portion 103 and the second slide portion 104 are electrically connected together via this sliding arm 102.
In the above construction, when the sliding arm 102 slides in accordance with the liquid level, the first contact portion 120 and the second contact portion 121 slide respectively over the first slide portion 103 and the second slide portion 104, and the first contact portion 120 contacts one of the first conductive segments 105 of the first slide portion 103, while the second contact portion 121 contacts one of the second conductive segments 107 of the second slide portion 104. A main resistance of an electric circuit formed by the first slide portion 103, the sliding arm 102 and the second slide portion 104 is a resistance of the resistor 105 lying between the end land 110a of the first slide portion 103 and the first conductive segment 105 with which the first contact portion 120 is contacted, and therefore a quantity of electricity corresponding to the sliding position of the sliding arm 102, that is, corresponding to the position of the liquid level, is obtained between the two end lands 110a and 110b. 
Here, when the first contact portion 120 of the sliding arm 102 contacting an arbitrary one of the first conductive segments 105 shifts into contact with its adjacent first conductive segment 105, the quantity of the resistance of the resistor 106 between the adjacent first conductive segments 105 increases or decreases in a stepping manner, and therefore a variation in the liquid level can be detected as a fine step-like change.
Incidentally, the above liquid level detection apparatus 100 is required to have a high precision, and in order to meet this requirement, the following processing has heretofore been applied to the resistance plate 101. Namely, the resistance plate 101, having the resistor 106 beforehand formed into a low resistance value, is produced, and the sliding arm 102 is attached to this resistance plate 101. Then, a closed circuit is formed between the end lands 110a and 110b of the first and second slide portions 103 and 104, utilizing the sliding arm 102 as shown in FIG. 6, and there is effected a trimming operation in which notches a are formed in the resistor 106 at regions between the first conductive segments 105 by a laser, thereby adjusting the resistance value, and by doing so, the resistor 106 between the adjacent first conductive segments 105 is formed into a desired resistance value.
At the time when the liquid level detection apparatus 100 of the above construction is mounted on an automobile so as to detect a liquid level (or height) of fuel within a fuel tank, end portions of conductors of wires from a detector (not shown) are fixed respectively to the end lands 110a and 110b of the resistance plate 101 by soldering in electrically connected relation thereto. Incidentally, in the resistance plate 101, the end land 110a serves as a plus (+) terminal, while the end land 110b serves as a minus (−) terminal, that is, a ground (GND) terminal. As described above, the liquid level detection apparatus 100 is so constructed that the detection output can be obtained between the end lands 110a and 110b of the resistance plate 101, and when the detector (not shown) obtains this detection output, a direct current flows from the end land 110a to the end land 110b, and therefore a potential difference develops between the end land 110a and the end land 110b. 
Incidentally, the above liquid level detection apparatus 100 is often used in a fuel tank of an automobile for holding as fuel an electrolyte (alcohol) (such as ethanol and methanol) itself or gasoline containing such electrolyte. Naturally, there are occasions when the fuel is brought into contact with that surface of the resistance plate 101 contacting the sliding arm 102 as when the resistance plate 101 is immersed in the fuel. In the resistance plate 101, needless to say, that portion thereof which is the largest in the potential difference toward the plus (+) side relative to the end land 110b (serving as the minus terminal) is the end land 110a. That portion of the resistance plate 101 which is the next largest in the potential difference toward the plus (+) side to the end land 110a to which soldering and coating are applied is a portion 111 (indicated by hatching) of the conductive layer extending between the end land 110a and the first conductive segment 105 closest to the end land 110a. Therefore, this portion 111 is liable to be most affected by electrolysis occurring as when the resistance plate is immersed in the fuel, and there is a high possibility that this portion 111 may be deteriorated and corroded. The potential difference between the end land 110a and the end land 110b logarithmically affects the degree of electrolysis.